1. Field of the Invention
The invention pertains to vibration absorbers of the bifilar centrifugal pendulum vibration absorbing type wherein the centers of mass of the absorber masses are restricted to move along prescribed paths relative to the rotating machine element whose oscillatory torques are to be absorbed.
2. Description of the Related Art
The invention pertains to a general class of vibration absorbers known as centrifugal pendulum vibration absorbers (CPVA) wherein centers of mass of the absorber masses, called pendulums, are restricted to move along prescribed paths relative to the rotating machine component whose vibrations are to be absorbed. Such masses, when properly designed, move in such a manner so as to remove torsional oscillations in the rotating machinery element by counteracting the applied torques which cause torsional oscillations. Such devices are used to dampen vibrations of internal combustion engine crankshafts, and helicopter rotors, and are typified by U.S. Pat. No. 3,372,758; 3,540,809; 3,874,818; 3,932,060; 4,057,363; 4,083,654; 4,218,187; 4,739,679 and 5,044,333.
In CPVA devices of the type previously developed, and as shown in the above mentioned patents, each pendulum or mass is tuned to counteract a torque component which is at a particular order of the rotation speed of the rotating element. For example, in an in-line four cylinder, four cycle automobile engine, the inertia of the pistons and the connecting rods and in-cylinder gas pressures produce dominant oscillatory torques which pulse with a frequency that is twice the engine's nominal rotational speed, which is a second order torque. Similarly, in a helicopter rotor application for a rotor having N number of blades, the rotor receives N torque pulses per rotation, for example as the rotor passes over the aircraft fuselage, and one order of the torque disturbance is therefore N. In order to use a CPVA to counteract an order N torque component, the CPVA is tuned so that its frequency of small amplitude oscillation is N Q, wherein Q is the nominal rate of rotation of the rotating element in radians per second. In this manner, the motion of the pendulums of the CPVA counteract the torque pulses. However, existing CPVA designs have several significant shortcomings.
First, with known CPVAs the actual frequency of oscillation of the CPVA generally shifts as the amplitude of pendulum oscillation increases, and this leads to serious problems and deficiencies of vibration absorption for certain ranges of torque amplitude and rotational speed. Modifications have been made to CPVA devices for modifying the absorber path to help this mistuning problems, such as shown in U.S. Pat. No. 4,218,187, however, such existing approaches to the problem have not completely overcome these deficiencies. Also, another correction tactic often employed is to intentionally mistune the absorber so that it comes into tune at some desired amplitude, but this procedure causes the absorber to be mistuned at all other amplitudes.
Secondly, even when frequency shifts are endeavored to be overcome by modifications of pendulum movement, such conventional approaches to the problem cause the CPVA to generate higher harmonics in the torque, especially in the 2N component, which reduces its effectiveness, and such approaches, while reducing the Nth harmonic amplitude by as much as 92% may only reduce the overall or net disturbance torque amplitude by 60%.
To our knowledge, no previous CPVA construction has been capable of the elimination of 100% of a torque harmonic over a wide range of operating conditions without inducing higher order harmonics.